The present invention generally relates to a circuit used in wireless communication systems, and more particularly pertains to a technology used in a multi-band low noise amplifier.
Multi-mode and multi-band receivers are important applications in recent development of wireless communication systems. A multi-mode receiver is capable of processing signals defined by various communication protocols, such as WCDMA protocol and GSM protocol. A multi-band receiver is capable of processing signals, which falls in various ranges of frequency bands. For example, a receiver included in a dual-band mobile phone is a typical multi-band receiver, which is capable of processing signals of 1.8 GHz and 900 MHz frequency bands. The frequency bands of signal to be processed by next generation WCDMA/GSM dual-mode receivers are 2.1 GHz, 1.9 GHz, 1.8 GHz and 900 MHz.
At present, wireless communication systems such as a treble band mobile phone, which comprises functions of PCS (GSM-1900 frequency band applied), DCS (adapt GSM-1800 frequency band applied) and GSM900 (adapt GSM900 frequency band applied), utilize several low noise amplifiers for processing multi-band signals. In such a treble band mobile phone, there are three low noise amplifiers applied in the RF chipsets included in the phone for receiving and configuring signals of three frequency bands, 1.9 GHz, 1.8 GHz and 900 MHz. It is considered to a feasible solution to integrate front-end components such as low noise amplifier and voltage controlled oscillator into RF chipsets for reducing the cost and simplifying the architecture of the RF chipsets. However, such integration may cause noise leakage from other circuits to the low noise amplifier and result in low signal quality. There are several solutions devised to overcome the problem and fully-differential circuit architecture is one of those solutions.
FIG. 1 illustrates a prior art fully-differential circuit applied in a low noise amplifier. The amplifier is a fully-differential circuit comprising: a differential pair 11, a common base transistor pair 12, a degenerating inductor pair 13, a load inductor pair 14, a current generator 15, and a matching network (not shown in the diagram). In the prior art, a fully-differential signal comes from an antenna going through the matching network, flows into the low noise amplifier, then output from collector of the common base transistor pair 12.
The advantages of the architecture are: (1) the impedance observed in base of the differential pair 11 is transferred to that required by the previous stage (usually is a band pass filter) by implementing degenerating inductor pair and input matching network so as to a power matching and a noise matching are accomplished, (2) the DC voltage across the load is reduced by applying load inductor, by doing so, not only required voltage is reduced and the linearity is enhanced, and (3) the working frequency of the circuit is increased by assigning appropriate load inductor for compensating parasitic capacitance of the output. On the other hand, the drawbacks of the architecture are: (1) inductors take up large area in a circuit, and (2) the architecture is better suited in narrow band applications. In other words, such a single low noise amplifier cannot be used for processing multi-band signals in a multi-band receiver. Therefore, in the implementation of a multi-band receiver, several low noise amplifiers are required. But, those low noise amplifiers take up a large area in the circuit. For example, the central frequency of WCDMA and DCS wireless communication systems is 2.14 GHz and 1.84 GHz respectively. There is a 300 MHz difference between two. As a result, if one chooses to apply one low noise amplifier illustrated in FIG. 1 for processing signals of two frequency bands, then the return loss and the noise figure are not acceptable for both bands simultaneously. Or, if one chooses to apply two low noise amplifier illustrated in FIG. 1 for processing signals of two frequency bands, then it means eight on-chip inductors will be required in two amplifiers and the cost of the product will accordingly increase.
Therefore, a new circuit architecture is needed, where components on the circuit can be shared without sacrificing the performance of the circuit so as to reduce the cost of a multi-band receiver. Solutions to overcome aforementioned problem are disclosed in several patents. In the U.S. Pat. No. 6,134,427, a control signal switch in a matching network 21 is applied for determining whether capacitor 23 and inductor 24 should function so as to generate a matching network 21 applicable for dual band signal processing. In the U.S. Pat. No. 5,995,814, an input matching network is utilized in the patented technology, where capacitor 32, 33 and inductor 34, 35 are used for ensuring the return loss for the dual frequency bands are in accordance with the specification of the communication protocol.
A low noise amplifier requires a large amount of on-chip inductors. A fully-differential circuit may require up to four on-chip inductors. At present, the area of an inductor may account up to 150 μm*150 μm, which explains the fact that utilizing several low noise amplifiers on a single chip does not only occupy a large area on the chip, also it results in the increase of the chipset cost. Therefore, it is desirable to find a solution resolving problems caused by the fact that an overly large area is taken up by inductors in the design of low noise amplifiers.